A METHOD FOR MEASURING MEAN LIFETIMES OF SHORT-LIVED (NANOSECOND REGION) MOLECULAR-IONS FORMED IN FAST COLLISIONS

The HeD2+ molecular ions are bound in their first excited electronic s tate, and decay to their repulsive ground state with a mean lifetime o f a few nanoseconds. These short lived molecular ions are produced by charge stripping collisions of 900 keV HeD+ ions in Ar gas. The ions e merging from a short target cell are analysed by a small powerful perm anent magnet. The trajectories of all reaction products are determined within a few nanoseconds. The simultaneous detection of both fragment s i.e. a ''fragment-fragment'' coincidence, peaking at the proper defl ection angle, is a clear signature of an HeD2+ molecular ion surviving through the analyzer. The average mean lifetime of HeD2+ was determin ed from the measured number of the ''fragment-fragment'' coincidences as a function of the distance between the target cell and the magnet. Similar measurements were performed to determine the mean lifetime of other HeH2+ isotopes.